Methods for inhibiting the polymerization of ethylenically unsaturated monomers

ABSTRACT

The polymerization of an ethylenically unsaturated monomer such as isoprene and butadiene is inhibited during storage or handling thereof by the addition of a dihydroxycinnamic acid or derivative thereof.

FIELD OF THE INVENTION

This invention relates to methods for inhibiting polymerization of ethylenically unsaturated monomers. The methods are particularly effective at inhibiting polymerization of these monomers during storage conditions in the presence of oxygen.

BACKGROUND OF THE INVENTION

It is well known that ethylenically unsaturated monomers readily polymerize when heated and that the polymerization rate increases with increasing temperature. For this reason, the monomers are stabilized by the addition of substances which act as inhibitors or retarders of polymerization, or which block the initiation mechanism leading to the form of radicals.

Certain ethylenically unsaturated (diolefin) monomers such as butadiene and isoprene polymerize when left in storage tanks and during transportation at temperatures as low as room temperature. This polymerization is initiated by reaction of the diolefin monomer with oxygen present in the monomer containing system. This reaction forms peroxides and free radical species which can further react with the diolefin monomer.

To prevent this polymerization from taking place, diolefin monomer manufacturers frequently add tert-butyl catechol (TBC) as a polymerization inhibitor. However, TBC is a toxic compound and can cause tissue damage when it contacts skin. These characteristics make it desirable to utilize an inhibitor compound that avoids these difficulties as well as being water and caustic extractable so that it could be easily removed from the finished product before use or additional processing.

The present inventor has discovered that dihydroxycinnamic acid and its derivatives effectively inhibit the polymerization of ethylenically unsaturated monomers during storage and transportation. These com- pounds prove particularly effective in the presence of oxygen and at temperatures up to 100° C.

DESCRIPTION OF THE RELATED ART

EP 89-402156 discloses a cosmetic preparation containing an antioxidant system based on a stabilized ascorbic acid ester. The an system also contains tocopherol or mixtures of tocopherol, caffeic acid or its derivatives, a complexing agent and a non-thiol polypeptide.

JP Kokai JP 59015477 teaches a food antioxidant composition effective for oils and fats which comprises 3,4-dihydroxycinnamic acid esters and one or more of ascorbic acid-stearic acid ester, citric acid, alic acid, and ascorbic acid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to methods for inhibiting the polymerization of ethylenically unsaturated monomers comprising adding an effective inhibiting amount of a dihydroxycinnamic acid or derivative thereof.

The dihydroxycinnamic acid or derivatives thereof generally have the formula: ##STR1## wherein R is H, CH₃ or C₂ H₅ ; R¹ is H, CH₃ or C₂ H₅ with the proviso that at least one of R¹ is H. The preferred compounds for use in the present invention are dihydroxycinnamic acid and dihydroxydihydrocinnamic acid.

The dihydroxycinnamic acid or derivatives thereof are effective at inhibiting the polymerization of ethylenically unsaturated monomers during their storage or under transportation conditions. These conditions are typically found to have oxygen atmospheres present and monomer temperatures up to about 100° C. The present inventor anticipates that the methods of the present invention can inhibit the polymerization of ethylenically unsaturated monomers during the processing thereof. These processing conditions, such as purification and distillation processes, typically employ heat and will often cause fouling of the monomer.

The ethylenically unsaturated monomers are characterized as polymerizable ethylenically unsaturated hydrocarbons and include olephins such as alpha olephins containing 2 to 20 carbon atoms a preferably 2 to 8 carbon atoms, and conjugated di-olephins, preferably those containing 4 to 6 carbon atoms such as isoprene and butadiene.

For purposes of this invention, the term "effective amount" refers to the amount of dihydroxycinnamic acid or derivative thereof necessary to inhibit polymerization of ethylenically unsaturated monomers. This amount will vary according to the conditions under which the ethylenically unsaturated monomer is subjected during storage and/or handling thereof. A higher temperatures and higher monomer contamination, larger amounts of polymerization inhibiting compound are generally required.

Preferably the effective amount of dihydroxycinnamic acid or derivative added to the ethylenically unsaturated monomer ranges from about 1 part to about 1000 parts per million parts of monomer. More preferably, the amount of dihydroxycinnamic acid or derivative thereof added to the monomer ranges from about 10 parts to about 200 parts per million parts monomer.

The dihydroxycinnamic acid derivatives may be used in combination with the dihydroxycinnamic acid or as a mixture of the dihydroxycinnamic acid derivatives when utilized in the methods of the present invention.

The dihydroxycinnamic acid or derivative thereof may be added to the ethylenically unsaturated monomer as either a dispersion or as a solution using a suitable liquid carrier or solvent. Any solvent that is compatible with the dihydroxycinnamic acid or derivative thereof and the ethylenically unsaturated monomer may be employed.

The invention will now be described with reference to a number of specific examples which are to be regarded solely as illustrative, and not as restricting the scope of the invention.

EXAMPLES

Testing was performed to determine the effectiveness of the present invention at inhibiting the polymerization of ethylenically unsaturated monomers.

Tests were run in a stainless steel pressure vessel fitted with a glass sample container and cover, a stem, a pressure gauge with a continuous recorder, and the appropriate valves and fittings. A solution of 10 mL of isoprene in 40 mL of heptane and the designated treatment were placed in the glass sample container and inside the pressure vessel. The vessel was closed, filled with oxygen at 100 psig and heated in a water bath at 100° C. The pressure was recorded continuously until a break point in the pressure-time curve (i.e., steepest slope of the curve) was reached. The induction time was calculated as the time elapsed between the placing of the vessel in the bath and the breaking point. The results of this testing are reported in Table 1.

                  TABLE I                                                          ______________________________________                                         Polymerization Inhibition Testing                                              Treatment   Dose (ppm)                                                                               Induction Time (min.)                                    ______________________________________                                         Blank       --        25                                                       TBC         10        40                                                       BHT         10        31                                                                   40        32                                                       DHHCA       10        85                                                                   25        144                                                      Blank       --        21                                                       TBC         40        90                                                       DHHCA       40        110                                                      DHCA        40        54                                                       Blank       --        25                                                       DHCA        50        102                                                                  100       127                                                      ______________________________________                                          TBC is tertbutyl catechol                                                      BHT is 2,6di-t-butyl-4-methylphenol                                            DHCA is 3,5dihydroxycinnamic acid                                              DHHCA is 3,4dihydroxyhydrocinnamic acid                                  

These results demonstrate that the compounds of the present invention are effective at inhibiting the polymerization of ethylenically unsaturated monomers under conditions approximating storage. The commercially available inhibitors, TBC and BHT proved less effective than the inventive compounds.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention. 

Having thus described the invention, what I claim is:
 1. A method for inhibiting the polymerization of isoprene comprising adding to the isoprene an effective polymerization inhibiting amount of a dihydroxycinnamic acid or derivative thereof having the formula ##STR2## wherein R is H, CH₃ or C₂ H₅ ; R¹ is H, CH₃ or C₃ H₅ with the proviso that at least one of R¹ is H.
 2. The method as claimed in claim 1 wherein said dihydroxycinnamic acid or derivative thereof is selected from the group consisting of dihydroxycinnamic acid and dihydroxydihydrocinnamic acid.
 3. The method as claimed in claim 1 wherein said dihydroxycinnamic acid or derivative thereof is added to isoprene in an amount ranging from about 1 part to about 1000 parts per million parts of monomer.
 4. The method as claimed in claim 1 wherein isoprene is exposed to oxygen atmospheres.
 5. The method as claimed in claim 1 wherein isoprene has a temperature ranging from ambient to about 100° C.
 6. A composition comprising isoprene and a dihydroxycinnamic acid or derivative thereof having the formula ##STR3## wherein R is H, CH₃ or C₂ H₅ ; R¹ is H, CH₃ or C₂ H₅ with the proviso that at least one of R¹ is H.
 7. The composition as claimed in claim 6 wherein said dihydroxycinnamic acid or derivative thereof is selected from the group consisting of dihydroxycinnamic acid and dihydroxydihydrocinnamic acid. 